Effect of Bi addition on the upconversion luminescence properties of NaYbF :Er.

In this study, Bi incorporation in NaYbF :Er lattice and its influence on upconversion luminescence properties have been investigated in detail using techniques such as temperature-dependent luminescence, Fourier transform infrared spectroscopy and X-ray diffraction (XRD). The study was carried out to develop phosphors with improved upconversion luminescence. From photoluminescence and lifetime measurements it is inferred that luminescence intensity from NaYbF :Er increases with Bi addition. The sample containing 50 at.% Bi ions exhibited optimum upconversion luminescence. Increased distance between Yb -Yb and Er -Er due to Bi incorporation into the lattice and associated decrease in the extent of dipolar interaction/self-quenching are responsible for increase in lifetime values and luminescence intensities from Er ions. Incorporation of Bi into NaYbF :Er lattice reduced self-quenching among Yb -Yb ions and this facilitated energy transfer from Yb to Er . This situation also explains decrease in the extent of temperature-assisted quenching of emission from thermally coupled H and S levels of Er . Based on Rietveld refinement of XRD patterns it was confirmed that a maximum of 10 at.% of Bi added was incorporated into the NaYbF :Er lattice and the remaining complex co-exists as a BiOF phase. These results are of significant interest in the area of development of phosphors based on Yb -Er upconversion luminescence.